Pyrolysis of organic compounds



United States Patent O PYROLYSIS OF ORGANIC COMPOUNDS John Lynda Anderson, Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application September 24, 1953, Serial No. 382,187

Claims. C1. 260-329) This invention relates to the pyrolysis of organic compounds and, more particularly, to the preparation of quinodimethane-type compounds by the pyrolysis of certain organic ring compounds of aromatic nature, i. e., aromatic compounds including heterocyclic compounds having aromatic properties.

The pyrolysis of aromatic compounds having aliphatic side chains is known to yield certain types of products, the particular products formed being dependent on the particular aliphatic side chains on the aromatic nucleus. Thus, aromatic compounds having two methyl groups in the para position with respect to each other, e. g., p-dimethylbenzene or 2,5-dimethylpyridine, are known to form quinoid type compounds, e. g., l,4-dimethylene-2,5- cyclohexadiene and 2,5-dimethylene-Z,S-dihydropyridine, respectively. On the other hand, the pyrolysis of ethylsu'ostituted aromatic compounds, e. g., ethylbenzene and p-ethyltoluene, are known to form the corresponding vinylsubstituted compounds, 6. g., styrene and p-methylvinylbenzene, respectively, by a dehydrogenation reaction.

An object of the present invention is to provide a process of preparing quinodimethane-type compounds. A further object is to provide a process of preparing certain quinodimethane-type compounds included in a new class of dialkylidene heterocyclic compounds having conjugated double bonds, such new class of compounds being claimed in application Serial No. 382,222, entitled Organic Compounds and filed of even date herewith jointly in the names of applicant and Hilmer Ernest Winberg. Other objects will be apparent from the description of the invention given hereinafter.

The above objects are accomplished according to the present invention by pyrolyzing an ethyl methyl-substituted organic ring compound of aromatic nature, i. e., of aromatic character, having an ethyl group and a methyl group attached to annular carbons which are separated from each other by two other annular carbons. In general, the pyrolysis is carried out at a temperature above about 725 C. and the reaction products of the pyrolysis are rapidly cooled.

It has been discovered that organic ring compounds as characterized above can be pyrolyzed to unexpected products. More specifically, the carbon-carbon bond of the ethyl substituent is cleaved and a demethanation takes place rather than the expected dehydrogenation. As a result, the products formed by the process of this invention are ring compounds having a methylene group attached to each of two annular carbons separated from each other by two other annular carbons. These are quinodimethane-type compounds and are capable of spontaneous polymerization to linear polymers.

A preferred manner of carrying out this process comprises passing the ethyl methyl-substituted aromatic compound through a reaction zone at a temperature of 725 C.-900 C., preferably 800 C.825 C., and rapidly cooling the reaction products of the pyrolysis, preferably to a temperature below about -50 C. and, when the quinoice dimethane-type compound formed is relatively unstable and it is desired to isolate it in monomeric form, to an even lower temperature.

The pressure at which the pyrolysis is carried out is not critical. However, it is preferred to use a pressure of about 20 mm. of mercury or less. Especially good results are obtained when the pressure in the system (measured at a point following the cold receiver) is maintained at less than one micron mercury pressure.

The reaction time, i. e., the length of time the ethyl methyl-substituted aromatic compound is subjected to the pyrolyziug temperature of 725 C. to 900 C., is not critical and can be varied over wide limits. When the pyrolysis is carried out under very high vacuum, i. e., at pressures of less than one micron of mercury, the reaction time is very short, e. g., of the other of one second or less. At higher pressures the reaction time can be longer. The longest reaction times are required when the pyrolysis is carried out at higher pressures and at lower temperatures in the above-specified range.

The pyrolysis can be carried out in reaction vessels of any inert material. The use of a filler or packing material in the reaction vessel is not essential but it is preferable in order to provide better heat transfer. Any inert granular material is suitable for this use. This reaction vessel is conveniently constructed in the form of a cylindrical tube and can be made of any inert, heat-resisting material such as, for example, heat resistant glass, quartz or inert metal.

Granular quartz is a satisfactory filler for the reaction tube. The reaction tube is heated externally by any convenient means, e. g., by an electrical coil heater.

The reaction products are led directly from the reaction zone into a cold receiver. The temperature at which the receiver is maintained, is dependent on the particular dimethylene-substituted product being prepared and on.

whether the product is desired in the monomeric or polymeric form. Generally, the receiver should be cooled to at least 50 C. to isolate products of normal stability while a temperature of liquid air or liquid nitrogen is required when less stable products are to be isolated in monomeric form. For example, in the pyrolysis of 5- ethyl-2-methylthiophene the monomeric product can be isolated in a receiver cooled to the temperature of a m'u'rture of solid carbon dioxide and acetone but when a less stable monomer is being isolated, e. g., 2,5-dimethylene- 2,5-dihydropyridine, in monomeric form, the receiver is preferably cooled by a liquid air or liquid nitrogen bath. In some cases, e. g., in the pyrolysis of p-ethyltoluene, the quinodimethane pyrolysis product is so readily polymerized even at the temperature of liquid nitrogen that only the polymeric product is obtained.

The less reactive monomers obtained in the process of this invention can be isolated and purified by distillation at low pressures. The polymeric products can be purified by conventional methods, e. g., by extraction with liquids which are solvents for impurities but not solvents for the polymer, or by solution in a liquid which is a solvent for the polymer followed by its crystallization from the solutron.

The following examples wherein all proportions are by weight unless otherwise'stated, illustrate specific embodiments of the invention.

Example I One ml. of p-ethyltoluene is distilled through a cylindrical quartz tube (1" x 12") filled with quartz packing and heated at 800 C., the pressure of the system being maintained at approximately one micron of mercury (measured between the vacuum pump and the cold receiver). The pyrolysis products are led through a trap held at liquid nitrogen temperature where they form a solid ring which cannot be distilled on subsequent warm- 3. ing. This solid material is poly-p-xylylene. The infrared spectrum and the X-ray .difilraction pattern of this product correspond closely with those for poly-p-xylylene prepared by the known method involving the pyrolysis of .p-xylene.

In a run similar to that of Example I except that the pyrolysis is carried out at 825 C., analogous results are obtained. The infrared spectrum of the solid product obtained is similar to that .of poly-p-xylylene prepared by pyrolysis of p-xylene. The elemental analysis of the product shows 91.25% carbon and 8.0% hydrogen (theory; 92.26%; 7.74% H).

In further runs similar to Example I except that the pyrolysis is carried out at 800 C. at 10 mm. mercury pressure, and at 750 C. in nitrogen at atmospheric pressure, analogous results are obtained, as shown by X-ray diffraction patterns of the solid polymeric reaction products.

Example 1] Using the same apparatus as in Example I, 0.5 ml, of S-ethyI-Z-methylpyridine is passed through the heated zone maintained at 800 C., with the pressure of the system maintained at less than one micron of mercury (measured between the vacuum pump and the co1d receiver). About one-half hour is required for the distillation. The products are condensed in the U trap cooled in liquid nitrogen and melt on removal of the liquid nitrogen coolant and begin to flow down the sides of the tube. Polymerization then takes place and a solid, light colored to colorless polymeric residue is obtained. This residue is soluble in dilute hydrochloric acid and is reprecipitated by the addition of a base in the same manner as a specimen of polymer obtained on pyrolysis of 2,5- dimethylpyridine. This polymeric residue is poly-2,5- dimethylene-2,5-dihydropyridine.

Example III One gram of S-ethyl-Z-methylthiophene is pyrolyzed at 825 C. in apparatus of the type used in Example I. The system is maintained at a pressure of less than one micron of mercury and the distillation of the 5-ethyl-2- methylthiophene through the system requires 30 minutes. The yellow condensate in the first trap, cooled by liquid nitrogen, melts on warming and is redistilled to an adjacent trap also cooled by liquid nitrogen. On this distillation a small amount of a dark residue remains in the first trap. The distilled product is remelted and redistilled in vacuum two more times. In both cases a small amount of colorless solid remains in the trap from which the liquid is distilled. On warming to room temperature the redistilled product polymerizes rapidly with the deposition of a colorless solid and the loss of the yellow color in the supernatant liquid. This solid is poly-2,5-dimethylene-Z,S-dihydrothiophene. It is partially soluble in hot hydrocarbon solvents, e. g., benzene, and is reprecipitated on cooling. In duplicate runs similar products give infrared absorption spectra which confirm the presence of the disubstituted thiophene structure. Elemental analyses confirm the empirical formula (CGHSSx).

Calculated: C, 65.5%; H, 5.45%. Found: C, 66.85%; H, 5.64%.

Poly-2,5-dimethylene-2,S-dihydrothiophene can be melt pressed into tough films.

It will be understood the above examples are merely illustrative and that the presentinvention broadly comprises the preparation of quinodimethane-type compounds by pyrolyzing an ethyl methyl-substituted organic ring compound of aromatic nature having an ethyl group and a methyl group attached to annular carbons which are separated from each other by two other annular carbons, generally at a temperature above about 725 C.

The instant process is applicable with the characteristic and unexpected demethanation mentioned above, to any ethyl methyl-substituted organic ring compound as defined above. Typical specific compounds other than pound of this type.

those of the examples, operable in this process include 5 -ethyl-2-methylpyrrole, 2-ethy1 5 methylthiophene, 1- ethyl-4-methylnaphthalene, and S-ethyl-S-methylquinoline.

It will be noted that the present process is adapted to form dialkylidene heterocyclic compounds having the formula wherein X is oxygen, sulfur, or NR, and R is hydrogen or an alkyl radical. Example III shows the preparation of 2,S-dimethylene-Z,S-dihydrothiophene, a specific com- These compounds have not been known heretofore and are included in the new class of dialkylidene heterocyclic compounds having conjugated double bonds, being claimed in the aforementioned application filed jointly in the names of applicant and Hilmer Ernest Winberg.

The products formed by the process of this invention are useful for various purposes. The monomeric compounds are useful as chemical intermediates, for example, in polymerization and for the preparation of monoand dihalogen derivatives by 1,6-addition of hydrogen halide or halogen, respectively. The monomeric compounds can also be converted to cyclic dimers by heating the monomers in the presence of a polymerization inhibitor, as described in application Serial No. 382,224, filed of even date herewith in the name of Hilmer Ernest Winberg and entitled Cyclic Dimers and Preparation Thereof. The polymeric products of this invention are useful in such applications as adhesives, coating compositions, and plastics.

A particular advantage of the present invention is that it provides a practical method of preparing a wide variety of useful quinodimethane-type compounds from available starting materials not heretofore known to be adapted for the preparation of such compounds.

As many apparently widely diflferent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in theappended claims.

The invention claimed is:

1. Process of preparing a quinodimethane-type compound which comprises pyrolyzing an ethyl methylsubstituted organic ring compound of aromatic nature having not more than two rings and having the ethyl group and the methyl group as the only substituents and attached to annular carbons of the same ring which are separated from each other by two other annular carbons.

2. Process of preparing a quinodimethane-type compound which comprises pyrolyzing at a temperature above 725 C. an ethyl methyl-substituted organic ring compound of aromaticnature having not more than two rings and having the ethyl group and the methyl group as the only substituents and attached to annular carbons of the same ring which are separated from each other by two other annular carbons, and cooling the quinodimethanetype compound formed to a temperature below about C.

3. Process as set forth in claim 2 wherein said organic ring compound of aromatic nature is pyrolyzed at a temperature of 725 C. to 900 C. and a pressure of less 6. The process which comprises pyrolyzing an ethyl methyl-substituted organic ring compound of aromatic nature having not more than two rings and having the ethyl group and the methyl group as the only substituents and attached to annular carbons of the same ring which are separated from each other by two other annular carbons.

7. The process of preparing a methylene compound which comprises pyrolyzing an ethyl methyl-substituted organic ring compound of aromatic nature having not more than two rings and having the ethyl group and the methyl group as the only substituents and attached to annular carbons of the same ring which are separated from each other by two other annular carbons.

8. The process which comprises pyrolyzing at a temperature above 725 C. an aromatic organic compound of up to two rings and having an ethyl group and a methyl group as the only substitucnts and attached to annular carbons of an aromatic ring, said annular carbons being separated from each other by two other annular carbons.

9. The process which comprises pyrolyzing at a temperature of at least 725 C. an organic compound of up to two rings and of aromatic character having an ethyl group and a methyl group as the only substituents and attached to annular carbons of an aromatic ring, said annular carbons being separated from each other by two other annular carbons.

10. The process which comprises pyrolyzing at a temperature of at least 725 C. an aromatic compound of up to two rings having as its only substituents an ethyl group and at least one methyl group, the ethyl group and one methyl group being attached to annular carbons of the same ring separated from each other by two annular carbons.

No references cited. 

1. PROCESS OF PREPARING A QUINODIMETHANE-TYPE COMPOUND WHICH COMPRISES PYROLYZING AN ETHYL METHYLSUBSTITUTED ORGANIC RING COMPOUND OF AROMATIC NATURE HAVING NOT MORE THAN TWO RINGS AND HAVING THE ETHYL GROUP AND THE METHYL GROUP AS THE ONLY SUBSTITUENTS AND ATTACHED TO ANNULAR CARBONS OF THE SAME RING WHICH ARE SEPARATED FROM EACH OTHER BY TWO OTHER ANNULAR CARBONS. 